It is known in automotive vehicle control systems to utilize wheel speed information which is processed in a computer for operating a control such as anti-lock brakes or traction control. Frequently the wheel speed of all four wheels or of both front wheels and the average of the rear wheels are used for control purposes. In such cases three or four variable reluctance wheel speed sensors are fed to the computer control, each sensor having a separate channel for processing the wheel speed. The processing may include sensor and channel diagnostics, amplification, square wave generation, A/D conversion, and determination of each wheel speed. At some point in the process the signals are digitized and fed to the computer.
To the extent that the channels are separate, it is relatively easy for the computer to diagnose faulty operation in a sensor or channel: when the wheel speeds are expected to be equal, the channel outputs are compared and any deviate channel has an output different from the others. Verifying the processing occurring within the computer is more difficult. It has been proposed to use two microprocessors running in parallel so that one can be a check for the other. However, such large scale redundancy is expensive in terms of the amount of space required on a computer die.
To upgrade anti-lock brakes and traction control systems, it is here proposed to include on the computer die functions which previously have been accomplished separately. Yet because of the need to conserve space on the silicon die, the processing must be done efficiently and in some cases with less circuitry than used in more conventional systems. One feature is to multiplex the incoming sensor signals and process the signals through only one channel instead of three or four. This presents some challenges, for example, diagnostics of the external sensors, harnesses and processing channel no longer can rely on comparing one channel to another. New diagnostics, then, are needed not only for external circuits but for internal operations. An advantage to incorporating the input functions on the same die as the computer is that the computer is readily available for assisting with some of the processing or the diagnostics, affording some amenities not previously practical.
At the outset, it is important to assure the integrity of the input signals which may be affected by shorts or open circuits in the sensors or the sensor harness or by noise introduced by the environment. Diagnostic circuitry can accomplish this during vehicle operation, however the additional question arises as to the integrity of the diagnostic circuitry. Other aspects of the control such as the speed processing module also require verification of integrity. The computer itself is subject to error and benefits from an integrity check.